Surface Structure of Alkyl/Fluoroalkylimidazolium Ionic–Liquid Mixtures

The gas–liquid interface of ionic liquids (ILs) is critically important in many applications, for example, in supported IL phase (SILP) catalysis. Methods to investigate the interfacial structure in these systems will allow their performance to be improved in a rational way. In this study, reactive-atom scattering (RAS), surface tension measurements, and molecular dynamics (MD) simulations were used to study the vacuum interface of mixtures of partially fluorinated and normal alkyl ILs. The underlying aim was to understand whether fluorinated IL ions could be used as additives to modify the surface structure of one of the most widely used families of alkyl ILs. The series of ILs 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Cnmim][Tf2N]) with n = 4–12 were mixed with a fixed-length, semiperfluorinated analogue (1H,1H,2H,2H-perfluorooctyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C8mimF13][Tf2N]), forming [Cnmim](1–x)[C8mimF13]x[Tf2N] mixtures, where x is the bulk mole fraction of the fluorinated component. The RAS-LIF method combined O-atom projectiles with laser-induced fluorescence (LIF) detection of the product OH as a measure of surface exposure of the alkyl chains. For [C8mim](1–x)[C8mimF13]x[Tf2N] mixtures, RAS-LIF OH yields are below those expected from stoichiometry. There are quantitatively consistent negative deviations from linearity of the surface tension. Both results imply that the lower-surface-tension fluoroalkyl material dominates the surface. A similar deficit is found for alkyl chain lengths n = 4, 6, 8, and 12 and for all (nonzero) x investigated by RAS-LIF. Accessible-surface-area (ASA) analyses of the MD simulations for [Cnmim](1–x)[C8mimF13]x[Tf2N] mixtures qualitatively reproduce the same primary effect of fluoro-chain predominance of the surface over most of the range of n. However, there are significant quantitative discrepancies between MD ASA predictions and experiment relating to the strength of any n-dependence of the relative alkyl coverage at fixed x, and on the x-dependence at fixed n. These discrepancies are discussed in the context of detailed examinations of the surface structures predicted in the MD simulations. Potential explanations, beyond experimental artifacts, include inadequacies in the classical force fields used in the MD simulations or the inability of simple ASA algorithms to capture dynamical factors that influence RAS-LIF yields.


Commercial ionic liquid characteristics
While most experiments were performed with ILs prepared in house (see main text), we also performed repeat RAS-LIF measurements at the end of the study on two ILs, [C8mim] [Tf2N] and [C12mim][Tf2N], supplied by by IoLiTec (Germany) and used as received. Table S1 shows the manufacturer's characterisation data for these liquids.

Liquid
Purity  Table S1: Ionic liquid purity information for commercial samples.

Run Parameters and Protocols
A 2 fs time step was used in conjunction with the leap frog integrator. Lengths of all bonds to H atoms were constrained. Shifted potentials with energy and pressure corrections were used with 1.5 nm cut-offs for the van der Waals and Coulomb potentials. The long-range electrostatic potential was calculated using particle-mesh Ewald with an order of 4 and minimum grid spacing of 0.12 nm. The data were saved every 2 ps and 4 ps for the small (800 ion pairs) and large (1600 ion pairs; n = 8, x = 0 and 0.5 only) simulations respectively.
The composition and dimensions of the systems studied are listed in Table S2. Each liquid was simulated following one of three protocols. The protocols varied slightly in run length for the bulk equilibration phase and in the slab heating and cooling cycles for the slab. Table S3 lists the protocols and the liquids to which they were applied. Tables S4 to S6 show the simulation parameters. The time constants for the barostat and thermostats are represented by τp and τt respectively.

ASA Threshold Dependence
The ASA method reports the surface area of each atom in the simulation frame. The algorithm was used with a probe-particle radius of 0.15 nm, corresponding to the van der Waals radius of an oxygen atom, and 10,000 dots for the resolution.
A small number of false atom counts from bulk voids were also detected by the ASA method.
For the purposes of carrying out preliminary tests of equilibration and convergence, etc, the false counts were eliminated by applying a surface area threshold, Tarea, before a hydrogen atom could be counted as being at the vacuum-liquid interface. An appropriate value was selected by examining the x = 0.5 to x = 1 ratio of secondary hydrogen atoms, (where the surface count of secondary hydrogens was averaged over the simulation run starting from 1 ns) as a function of threshold area, for the C8 system. This is shown in figure S1 for different simulation run times. At low Tarea, there is a slow decline in the ratio until between 0.15 -0.20 nm 2 , where the ratio does not vary significantly. With larger values of Tarea, the ratio can diverge causing an initial increase in the ratio before reaching zero as Tarea exceeds the area that a fully exposed atom presents to the probe particle. This analysis was also applied to the larger 1600 ion pair system, and to the x = 0.25 C4 mixture, with similar results. Therefore, a threshold value of 0.19 nm 2 was arbitrarily chosen when calculating the surface H-atom count for equilibration tests between runs.
For the subsequent, more-detailed quantitative comparison of the exposures of different atom types, including different positions along alkyl chains, the total exposed area of each type was summed, without applying any threshold. Although, as noted above, this admits a very minor contribution from atoms which are not truly at the surface, it avoids more significant systematic distortions resulting from differential sensitivities of different atom types to the choice of the threshold area.

Block averaging
A block analysis 1 was used to determine the relaxation times over several 320 K liquid runs.
The ASA values (thresholded secondary-H atom count) were calculated from 1 ns after the start of the run to allow the surface to relax from 500 K. The simulation run is divided into blocks of variable length, b, so that the total number of blocks in a simulation run is nb, where T is the total length of the simulation measured in saved data points. The mean of each block, 〈 〉 , is calculated according to where Ai is the ASA value from a single frame, i, in the simulation and 〈 〉 is the average for a single block of length, b. The variance of the mean for each block from the overall mean is calculated, and then averaged according to where 2 (〈 〉) is the averaged variance of the blocks for a given block length, b, 〈 〉 is the mean ASA value over the all of the frames in the simulation (excluding the first 1 ns). The statistical inefficiency was calculated as   The runtime dependence of the exposed secondary-hydrogen exposed atom count,  figure S7. We conclude that there are no significant differences between the large and small systems. Also shown in Figure S8 is the runtime dependence for a selection of other liquid mixtures of different composition and chain length, as indicated. In all cases, and as in Fig. S7, results are stable generally, and with S17 confidence beyond ~45 ns. This is the basis for choosing to analyse surface composition beyond this time to obtain average quantities, as stated in the main text. There are also no significant differences between averaging every frame ('total mean'), or (much more efficient) sampling every 400 ps, as derived in Section S2.3 and adopted for the analysis of production runs.

MD Areas
ASA-determined areas for different liquids are tabulated below.

Surface tension results [C8mim](1-x)[C8mimF13]x[NTf2]
x  for x = 0 and x = 0.5. The wavelength step size was 0.0020 nm and data was collected for 40 laser shots at each step. The photolysis-probe delay was fixed at 13 µs corresponding to the peak of the appearance profile. For x = 0, seven wavelength scans were recoded. For x = 0.5, since there was less OH signal from this mixture, twelve scans were recorded. Spectra were recorded on the Q1 branch only up to N = 5, where N is the sum of the electron orbital quantum and rotational quantum numbers.

Rotational analysis
LIFBASE 3 was used the fit the spectra for both liquids, the resultant populations were normalized over the five rotational levels recorded and averaged over the number of scans taken for each mixture. Figure S10 shows the normalized Q1 branch populations of OH